Rosetta: Closing in on the comet Print
Tuesday, 19 August 2014 10:33

The Rosetta spacecraft is now in the immediate vicinity of the comet nucleus to be explored, the one of Comet 67P/Churyumov-Gerasimenko. This target will soon be mapped systematically, in great detail, as part of the preparations for the touchdown of the Philae lander, foreseen for November this year. However, the OSIRIS cameras have already delivered quite astounding pictures of the weird object, and we are as yet far from understanding exactly what we are looking at. Here are some examples of the riddles that have been posed and the scientists of the camera team are working on.

The spin of the nucleus

Early in the approach phase, before resolved images of the nucleus could be taken, its brightness could be monitored. Since this showed a periodic variation, one could conclude that it was spinning around an axis and that its shape was far from spherical. But now came the first surprise! The period of the spin was determined to be about 12.4 hours, markedly different from what had been inferred from similar light curve observations performed from the Earth 6-7 years ago, when the comet was in a similar part of its orbit. At that time, the result was 12.7 hours.

Was there an error in the analysis? No, apparently not. The nucleus seems to have spun up significantly during its last orbit around the Sun. In view of its strange shape, the jet effect of the gas stream emerging from the nucleus when it is closer to the Sun may well induce a torque that is large enough to change the spin of the nucleus. One question then arises. If you interfere with a spinning object, it naturally starts to wobble, which means that the axis of rotation is not fixed but turns in space. But we do not see such a wobbling motion in the nucleus of our comet. Apparently, such a motion is damped out very quickly.

So, what does this tell us about the nucleus? Perhaps it is not a unique, solid block of material but is built of different parts that just barely stick together, and there may be large voids inside it. In such a case, a wobbling could easily be damped out. But this is just a hypothesis, and future measurements by Rosetta will be very important to find out, if it is true. In particular, we attach much hope to the CONSERT experiment, which will attempt to perform radio tomography of the nucleus. This may tell us about its global structure and how the mass is distributed within the object.

The issue of mass and gravity

Nine years ago, two of the OSIRIS team members – Björn Davidsson and Pedro Gutiérrez – published a paper, where they analyzed the data that were then available about the production rates of water vapor and the jet force that was influencing the orbital motion of comet Churyumov-Gerasimenko. They made a tentative conclusion about how the spin axis was oriented in space, how the water production was distributed around the nucleus, and finally, about the mass of the comet and its density. This was a bold prediction, and naturally it was met by some skepticism by comet scientists in general. But the conclusion about the spin axis was supported by what others had found using different techniques.

Now the predictions by Davidsson and Gutiérrez are facing their final verdict, since they can be tested by the findings of Rosetta. The first test concerned the spin axis, and this was successful. From the OSIRIS images, we clearly see that the nucleus is spinning just the way most scientists had inferred. This removes some of the doubts about the estimate of the mass, but the issue will soon be settled by the direct measurements made using the radio signal from Rosetta, and what they tell about how the motion of the spacecraft is influenced by the gravity of the nucleus.

One thing can already be said with confidence. If the mass estimate is correct, then the centrifugal force implied by the rotation of the nucleus is quite large compared to the force of gravitational attraction. This means that the effective gravity on the nucleus surface is very small, and there may even be places where it drops to zero. Obviously, such places had better be avoided when delivering the Philae lander, so this part of the exploration is very important.

The nucleus – a curious object!

Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The images taken by the OSIRIS cameras have shown the nucleus in better and better resolution, and it looks more and more curious! Currently, the best pictures show features just several meters in size. Let us look at the two images shown here. The upper one was downloaded from Rosetta on 6 August, and it shows what looks like two independent parts connected by a waist-like region in the middle. There are a fantastic variety of surface features on both parts, and at first glance they look rather similar to each other in their ragged appearance, while the waist looks smoother.

As usual, such pictures cause people to relate the cosmic object to experience from everyday life. Some have likened this comet nucleus to a duck, and some to a hiking boot. But the big question is what has created this strange shape? We have to figure out, what is most likely: that it is two different objects that once came together in a low-velocity collision so that the waist region shows the merger, or that it is one object from which the waist has been carved out due to the sublimation of ice during tens of thousands of years in the past. This is not easy to tell, because the orbit of the comet is not stable, and the changes cannot be traced for more than a few centuries. But whether or not the nucleus may have been shaped by erosion of the ice can probably be judged on a statistical basis, and we are looking into this issue.

credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The lower image is even more recent and features the two parts as seen from underneath the sole of the boot. The shadow cast by the closer part on the waist is very evident. The waist surface looks very smooth but part of it is strewn by strange protrusions that look vaguely like boulders but may rather be local leftovers of ice sublimation. This remains to be studied on the detailed images to be taken as the comet approaches the Sun, and the sublimation gains speed. On the other, more remote part of the nucleus one can see a series of parallel ridges, whose origin remains unknown. Many crater-like features are also seen, especially beneath the sole, and they were most likely formed by local erosion due to ice sublimation.

So, can the ice be seen on the images? The surface is overall quite dark, a bit like the black boards of old-style schools. While this is not what one expects of pure ice, it does not exclude the presence of ice. But other evidence points in the direction of localized icy spots situated in a generally dusty environment. The OSIRIS pictures show jets of dust leaving the nucleus from specific regions, which seem concentrated in the waist area. And there are bright spots, whose color is more bluish than the rest of the surface – something that may indicate ice on these parts of the surface. We are still working on this, and I expect to write more later on, so stay tuned!

 

 

 

 

Hans Rickman

August 15, 2014

 

 

 

 

 

 

 

Last Updated on Wednesday, 20 August 2014 11:49